Implants are utilized in modern medical technology in a variety of applications. They are used for example, to support vessels, hollow organs, and ductal systems (endovascular implants e.g. stents), to fasten and temporarily fix tissue implants and tissue transplants in position, as well as for orthopedic purposes such as pin, plate, or screw and other applications. The stent is a form of an implant that is used particularly frequently.
Stent implantation has become established as one of the most effective therapeutic measures for treating vascular disease. Stents are used to provide support in a patient's hollow organs. To this end, some stents have a filigree support structure composed of metallic struts; the support structure is initially present in a compressed form for insertion into the body, and is expanded at the application site. One of the main applications of stents of this type is to permanently or temporarily widen and hold open vasoconstrictions, in particular constrictions (stenoses) of the coronary arteries. In addition, aneurysm stents are known, for example, which are used primarily to seal the aneurysm. They also perform the support function.
Some stents include a circumferential wall having a support force that suffices to hold the constricted vessel open to the desired extent; stents also include a tubular base body through which blood continues to flow without restriction. The circumferential wall can be formed by a latticed support structure that enables the stent to be inserted, in a compressed state with a small outer diameter, until it reaches the constriction in the particular vessel to be treated, and to be expanded there, e.g. using a balloon catheter, to the extent at which the vessel has the desired, increased inner diameter. Alternatively, materials having a memory effect, such as Nitinol, are capable of self-expansion in the absence of a restoring force that holds the implant at a small diameter. The restoring force can be exerted on the material by a protective tube.
The implant, in particular the stent, has a base body composed of an implant material. An implant material is a nonliving material that is used for a medical application and interacts with biological systems. A prerequisite for the use of a material as an implant material that comes in contact with the body environment when used as intended is its biocompatibility. “Biocompatibility” refers to the capability of a material to evoke an appropriate tissue response in a specific application. This includes an adaptation of the chemical, physical, biological, and morphological surface properties of an implant to the recipient tissue, with the objective of achieving a clinically desired interaction. The biocompatibility of the implant material is furthermore dependent on the time sequence of the response of the biosystem in which the implant is placed. For example, irritations and inflammations, which can cause tissue changes, occur over the relative short term. Biological systems therefore respond differently depending on the properties of the implant material. Depending on the response of the biosystem, implant materials can be subdivided into bioactive, bioinert, and degradable/resorbable materials.
Implant materials include polymers, metallic materials, and ceramic materials (as coating, for example). Biocompatible metals and metal alloys for permanent implants contain e.g. stainless steels (e.g. 316L), cobalt-based alloys (e.g. CoCrMo casting alloys, CoCrMo forging alloys, CoCrWNi forging alloys, and CoCrNiMo forging alloys), pure titanium and titanium alloys (e.g. CP titanium, TiAl6V4 or TiAl6Nb7), and gold alloys.
It is furthermore known that a greater level of biocompatibility can be achieved by coating implant materials with particularly tissue-compatible materials. These materials are usually organic or synthetic-polymeric in nature and are partially of natural origin. Further strategies for preventing restenosis focus on inhibiting proliferation using medication e.g. treatment using cytostatic agents. The active ingredients can be provided e.g. on the implant surface in the form of a coating.
The use of biocorrodible magnesium alloys for temporary implants having filigree structures is made difficult, in particular, by the fact that the implant degrades very rapidly in vivo. Various approaches to reducing the rate of corrosion, i.e. the degradation rate, are under discussion. For example, attempts are being made to slow the degradation of the implant material by developing alloys for this purpose.